The 1/4" steel by itself may be more flexible in bending than the whole 3/4" MDF top, but that's not really the comparison that matters. What matters is whether the steel is stronger/stiffer than the small section of MDF you're removing (i.e. the 1/4" deep channel you'd rout out). I'll leave it to others as to whether this is actually suitable for the application. My engineering gut reaction is that once you laminate the granite top to the rough top with adhesive, that this steel won't do much...its right at the center of the lamination, where there's very little bending stress anyway.
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macNov 19 '12 at 23:00

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For straps that wide, it would require 3/8" or 1/2" steel. As you've found, 1/4 hot rolled steel just isn't that stiff.
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Fiasco LabsNov 20 '12 at 15:25

3 Answers
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Based on the flexibility of the flat bar stock, I decided to use square 1/2" tube stock.

Using a 3/4" MDF subtop, I routed channels 5/8" wide and 9/16" deep. I then filled these channels with polyurethane glue, set in the steel bars, and finally I used a drywall knife to spread and flatten glue over the top of the bars.

Here are pictures of my actual work:
The cabinets are backed by a 2x8 pony wall. The MDF is glued and screwed to the pony wall and all cabinet frames. The steel bars span everywhere except the sink cabinet.

How much of an overhand do you plan on having? The spec I saw was that 10 inches were allowed with no supports. When the granite guy came to measure mine he told me I didn't really need them even at 12. I had alread put in corbels which look good, but do get in the way of knees. If I did it again, I would consider going to 10" and not having any bracking.

If the steel is epoxied to the granite, it will help, but I don't see the strips as shown doing that much good.

I have seen examples of granite on knee walls where a 1/4 inch plate spanned almost the entire granite slab.

If the strip steel is epoxied to the bottom surface of the granite, it'll do almost nothing. The bottom of the granite is exposed to compressive stress due to the bending, and granite is very good at resisting compressive stress all by itself. To help with the bending, you either need to make a sub-top that takes all the bending load by itself; or you'd need to laminate something with good tensile strength to the TOP of the granite top (where the tensile load is), which I don't think too many customers would be happy about.
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macNov 20 '12 at 16:32

@mac the goal is to add rigidity to the subtop. As implemented, the steel resists flex in the MDF (tensile at the top, compressive at the bottom)
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MatthewNov 20 '12 at 17:01

but once you've laminated the granite to the subtop, the top of the subtop isn't the top of the composite member anymore---it's the middle.
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macNov 20 '12 at 17:08

@mac I understand, but a rigid subtop helps prevent stress on the stone. Otherwise, why use a subtop?
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MatthewNov 20 '12 at 19:27

I don't know much about countertop weights, I've never done one, but I can tell you that flat steel is strongest along it's smallest edge. It's not supposed to be too rigid when flat. For example, if I have a quarter inch thick piece of steel, 3 inches wide and 1 foot long, if I put it down flat on 2 end supports and stood on it, it would bow. If I braced the sides and put my body weight on the quarter inch thick part, it wouldn't bow, there's 3 inches of steel supporting me instead of quarter inch.

I would recommend angle steel for you here. Route it, not plane. Angled steel is strong along both sides. You can get angle iron at most home improvement stores, just route out a deep channel for the other leg of the steel. Another thing that's pretty strong is hollow round stock, that may be easier for you to work with, and the arches help a lot.

Naturally, the strength will be along the smallest edge (as an i-beam) but I am limited in thickness to that of my subtop. Perhaps I'll route in square-tube stock rather than flat bar stock.
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MatthewNov 19 '12 at 23:20